The practice of bypassing a regulator is fairly common. Bypassing is done in order to avoid power disruptions when installing or removing a regulator from service. If it is not done properly, i.e.—the regulator is bypassed while the tap changer is not in the neutral position (commonly referred to as “Bypass off Neutral”), serious damage can result. When the tap changer is not in the neutral position, a voltage exists between the source and load bushings of the regulator. Bypassing the regulator creates a short circuit between the source and load bushings through the bypass switch. If the series winding has not been taken out of the circuit by moving the tap changer to the neutral position, the voltage across the source and load bushings can drive a very large current through the regulator series winding and bypass switch. This large current can burn insulation, create arcing, melt windings, and lead to a rupture of the regulator tank. Because of the typically small number of series turns involved, the ratio of series turns to shunt turns can be very small. This means that even though a very large bypass current is flowing in the series winding, a much smaller current is reflected into the shunt winding. This current can be near or below rated load current. As a result, upstream protection may, be unable to detect the situation until a ground fault occurs. Therefore, the protective equipment upstream of the device often cannot sense and/or cannot respond quickly enough to prevent the failure from becoming catastrophic.
Traditionally, the method for ensuring a safe bypass operation is a manual process in which the user is recommended to verify that the regulator tap changer is in the neutral position and no voltage differential is present between the load and source sides of the bypass switch and voltage regulator. Typically, such verification includes four possible methods: 1) verify that a neutral indicator light on the control is indicating the neutral position; 2) verify that the tap position display on the regulator control interface indicates the neutral position; 3) verify that the mechanical position indicator on the regulator is in the neutral position; and 4) verify by measurement that there is no voltage difference between the source and load bushing. Such methods are typically dependent upon the observation, judgment, knowledge, and conscientiousness of the user. Thus, such existing methods can be prone to human error.